The present invention relates to a method and apparatus for determining the deposition of particles in the respiratory tract of an individual, and/or for monitoring the function of the respiratory tract, by means of gas or aerosol pulses and measuring signals obtained from both the inhalate and exhalate streams of the individual.
Within the general examination of biological-medical influences of environmental substances, the exposure to contaminants by way of inhalation plays an important part. In this connection, two important partial aspects must be examined.
(a) What proportion of the particulate matter contained in the breathing air is deposited in the human respiratory tract and what are the parameters on which this depends? Such results are important for the initiation of suitable preventive measures.
(b) How can pathological changes in the respiratory tract as a result of prolonged exposure to contaminants be detected in time?
The first studies to examine the deposition of aerosol particles in the lungs employed the filter technique. Separate filter samples are taken from the inhaled and exhaled air of an individual and the particle concentration on the two filters is compared. However, in these processes, uncontrollable particle losses in the collecting and sampling systems may considerably falsify the particle deposition in the lungs as it is to be measured. A significant advance was made when, according to a proposal by B. Altshuler et al., A.M.A. Arch. Ind. Health (1957) Vol. 15, page 293, the concentration in the number of particles in the inhaled and exhaled breathing air was measured in a photometer directly in front of the mouth of the person being examined and was recorded continuously. From the volume curve simultaneously recorded by an expirograph, graphical integration then yielded the ratio of the number (N) of exhaled to inhaled particles, N.sub.e /N.sub.i. A subsequent article by J. Heyder et al., "Experimental Studies of the Total Deposition of Aerosol Particles into Human Respiratory Tract", Aerosol Science (1973) Vol. 2, pages 191-208 then taught changing the integration to an electronic base by continuously measuring, in addition to the particle concentration c (by photometry), also the volume stream V of the breathing air (by pneumotachography). There then results the following equation for the ratio of exhaled to inhaled number of particles per breath: ##EQU1## where t.sub.e is the duration of exhalation and t.sub.i is the duration of inhalation. In spite of the continuous recording of the concentration of the aerosol particles in front of the mouth of the test person and automation of the evaluation, the so-called analog method for determining a number of particles N according to equation (1) still has the following inherent drawbacks:
(1) It is limited to monodispersed aerosols. Each background of another, undesirable size fraction contributes to the values obtained from the photometer.
(2) Particles having hygroscopic surface layers attract water in the respiratory tract, and thus change their optical properties. Consequently, the photometer can no longer distinguish between a change in the number concentration c and a change in the scattering properties of the particles.
(3) In order to be able to make a continuous recording, it is necessary to have a relatively high particle concentration.
In addition to the standard clinical methods for performing lung tests with body plethysmographs (lung volume, airway resistance) it has recently become customary to make the patient inhale inert gases or short pulses of such gases and to draw conclusions as to the ventilation behavior of the lungs and thus to possible pathological changes from the wash out curves or the dispersion, respectively, of these gas pulses in the exhalate.
Such methods require continuous measurements of gas concentrations in the breathing air of test patients. In the past, three major physical principles have been used for such measurements:
1. mass spectroscopy;
2. infrared absorption at specific absorption lines of the gases;
3. changes in the thermal resistance in an environmental gas with changed thermal conductivity.
However, all these methods have the drawback that they do not take concentration measurements of the gas directly in the main stream of the breathing air. Rather, a representative sample must continuously be taken thus adversely influencing the time and volumetric resolution of the concentration curve.